This solution addresses Key issue #1.
When the UE is about to become unavailable due to the events as described in Key issue #1, the UE initiates registration procedure indicating unavailability period.
The AMF, may set the periodic registration timer to a value equal or slightly larger than the unavailability period provided by the UE.
During this unavailability period the 5GC maintains the UE context in CM-IDLE, and considers the UE unreachable, in similar approach as MICO mode.
When the UE concludes the event that kept it unavailable, the UE triggers registration procedure and does not provide indication of unavailability period. During that registration procedure, if the AMF had assigned a periodic registration timer based on unavailability period, the AMF provides the UE with a regular value for the periodic registration timer (i.e. not considering unavailability period).
The use of periodic registration timer of similar value of the unavailability period is to make sure the UE resumes normal activity after the unavailability period is concluded.
shows the call flow for this solution.
During initial Registration procedure, the UE provides indication of support of "Unavailability Period" in Registration Request, and the AMF provides support of "Unavailability Period" in Registration Accept message.
When an event as described in Key issue #1 is to be triggered in the UE, if "Unavailability period" support was indicated by AMF, a supporting UE proceeds with step 3 before executing the event and becoming unavailable.
The UE sends Registration Request message indicating an Unavailability period. The Unavailability period is determined internally in the UE, and implementation dependent, (e.g. may be determined based on type of event and expected event duration).
The AMF responds with Registration Accept. The AMF may set the periodic registration timer to a value equal or slightly larger than the unavailability period provided by the UE. The AMF stores the information that the UE is in unavailability period in UE context, and considers the UE unreachable until the UE performs registration procedure again. In this state, all HLCom solutions apply if supported, e.g. extended data buffering, downlink data buffering status report, etc.
Once the event is executed, the UE triggers registration procedure to resume regular service. The UE does not include Unavailability period in the registration request. Depending of the state the UE ends up after the event, the registration type may be initial registration, "mobility" or "periodic" registration.
The AMF responds with registration accept. If the AMF has stored that the previously assigned periodic registration timer was assigned based on unavailability period, the AMF assigns a new periodic registration timer not considering unavailability period.